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Bobo is a Faceted Search implementation written purely in Java, an extension of Apache Lucene
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/**
 * Bobo Browse Engine - High performance faceted/parametric search implementation
 * that handles various types of semi-structured data.  Written in Java.
 *
 * Copyright (C) 2005-2006  John Wang
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
 *
 * To contact the project administrators for the bobo-browse project,
 * please go to https://sourceforge.net/projects/bobo-browse/, or
 * send mail to [email protected].
 */

package com.browseengine.bobo.util;

import java.io.Serializable;
import java.lang.reflect.Array;

public abstract class PrimitiveMatrix implements Serializable {
  /**
   *
   */
  private static final long serialVersionUID = 1L;
  protected Object _matrix;
  protected int _rowCount;
  protected int _colCount;
  private int _growth;
  private int[] _dim;
  private transient Class _type;

  private static final int DEFAULT_SIZE = 1000;

  protected PrimitiveMatrix(Class type, int[] sizes) {
    super();
    _type = type;
    if (sizes == null || sizes.length != 2) throw new IllegalArgumentException(
        "Invalid dimension specified.");

    if (sizes[0] <= 0 || sizes[1] <= 0) throw new IllegalArgumentException(
        "Invalid size specified.");

    _matrix = Array.newInstance(type, sizes);
    _rowCount = 0;
    _colCount = 0;
    _growth = 10;
    _dim = sizes;
  }

  protected PrimitiveMatrix(Class type) {
    this(type, new int[] { DEFAULT_SIZE, DEFAULT_SIZE });
  }

  protected synchronized void expandRows(int upTo) {
    if (upTo <= _dim[0]) return;

    int old = _dim[0];
    _dim[0] = upTo * _growth;
    Object newMatrix = Array.newInstance(_type, _dim);
    for (int i = 0; i < old; ++i) {
      Object oldRow = Array.get(_matrix, i);
      Object newRow = Array.get(newMatrix, i);
      System.arraycopy(oldRow, 0, newRow, 0, _dim[1]);
    }
    _growth *= 10;
    _matrix = newMatrix;
  }

  protected synchronized void expandCols(int upTo) {
    if (upTo <= _dim[1]) return;
    int old = _dim[1];
    _dim[1] = upTo * _growth;
    Object newMatrix = Array.newInstance(_type, _dim);
    for (int i = 0; i < _dim[0]; ++i) {
      Object oldRow = Array.get(_matrix, i);
      Object newRow = Array.get(newMatrix, i);
      System.arraycopy(oldRow, 0, newRow, 0, old);
    }
    _growth *= 10;
    _matrix = newMatrix;
  }

  protected synchronized void expand(int rUpto, int cUpTo) {
    int[] newDim = new int[] { rUpto + _growth, cUpTo + _growth };
    Object newMatrix = Array.newInstance(_type, newDim);
    for (int i = 0; i < _dim[0]; ++i) {
      Object oldRow = Array.get(_matrix, i);
      Object newRow = Array.get(newMatrix, i);
      System.arraycopy(oldRow, 0, newRow, 0, _dim[1]);
    }
    _matrix = newMatrix;
    _dim = newDim;
    _growth *= 10;
  }

  public void ensureCapacity(int x, int y) {
    _rowCount = Math.max(x, _rowCount);
    _colCount = Math.max(y, _colCount);

    if (_rowCount >= _dim[0] && _colCount >= _dim[1]) {
      expand(x, y);
    } else if (_rowCount >= _dim[0]) {
      expandRows(x);
    } else if (_colCount >= _dim[1]) {
      expandCols(y);
    }
  }

  /**
   * called to shrink the array size to the current # of elements to save memory.
   *
   */
  public synchronized void seal() {
    if (_dim[0] > _rowCount || _dim[1] > _colCount) {
      int[] newDim = new int[] { _dim[0], _dim[1] };
      Object newMatrix = Array.newInstance(_type, newDim);
      for (int i = 0; i < newDim[0]; ++i) {
        Object oldRow = Array.get(_matrix, i);
        Object newRow = Array.get(newMatrix, i);
        System.arraycopy(oldRow, 0, newRow, 0, newDim[1]);
      }
      _matrix = newMatrix;
      _dim = newDim;
    }
    _growth = 10;
  }

  @Override
  public String toString() {
    StringBuffer buffer = new StringBuffer("{");
    for (int i = 0; i <= _rowCount; ++i) {
      if (i != 0) {
        buffer.append('\n');
      }
      Object row = Array.get(_matrix, i);
      for (int j = 0; j <= _colCount; ++j) {
        if (j != 0) {
          buffer.append(", ");
        }
        buffer.append(Array.get(row, j));
      }
    }
    buffer.append('}');
    return buffer.toString();
  }
}